1. Field of the Invention
The present invention relates to a method of producing a semiconductor device, and more particularly to an improved method of forming an insulating layer having a gradually sloped contour, especially, a contact hole with a roundish edge.
2. Description of the Prior Art
A semiconductor device, for example a metal oxide semiconductor field effect transistor (MOS FET) type integrated circuit (IC), is frequently provided with a phosphosilicate glass (PSG) layer as an insulating film. The PSG layer exhibits a passivation effect, e.g. an effect of trapping cations (e.g. Na ions), and useful plastic flowability at a high temperature. For example, a method of producing a semiconductor device using the plastic flowability of the PSG is disclosed in U.S. Pat. No. 3,825,442.
Usually, a PSG layer containing a high concentration of phosphorus is formed on a semiconductor substrate, a contact hole (i.e. a through hole) for an electrode is formed in the PSG layer, and the semiconductor substrate is heated at a high temperature to soften the PSG layer and round off the edges of the contact hole. When a conductive layer (i.e. an electrode) is formed within the contact hole and on the PSG layer, the round edge of the contact hole prevents discontinuity in the conductive layer at the edge.
FIG. 1 shows a part of a semiconductor device produced by using the plastic flow of the PSG layer. The device comprises a semiconductor (silicon) substrate 1 having an impurity doped region 2, a field oxide layer 3, a PSG layer 4 having a contact hole, a conductive (electrode) layer 5, and a covering 6 of PSG having an opening 7 for a bonding pad.
The semiconductor substrate 1 with the PSG layer 4 is heated to close to the softening point of the PSG, prior to the formation of the conductive layer 5, to cause plastic flow of the PSG layer at the edge of the contact hole, for connecting the conductive layer 5 to the impurity doped region 2, to round off the edge, as illustrated in FIG. 1. This makes it possible to prevent discontinuity of the conductive layer 5 at the edge of the contact hole.
It is preferable to carry out the heating treatment at as low a temperature as pssible in order to prevent deterioration of the semiconductor device caused by an elevated temperature (i.e., undesirable expansion of impurity doped regions formed in the semiconductor substrate; crystalline defects of the semiconductor substrate caused by thermal stress).
The higher the phosphorus concentration of the PSG, the lower the softening point of the PSG. For example, when the PSG has a high concentration of phosphorus of 1.times.10.sup.21 atoms/cm.sup.3, the heating treatment for plastic flow of the PSG is carried out at approximately 1000.degree. C. Therefore, a PSG containing a high concentration of phosphorus is usually used.
However, the higher the concentration of phosphorus, the larger the moisture absorption (hygroscopicity) of the PSG. When the PSG layer takes up moisture, the moisture reacts with the phosphorus to generate an acid. Acids gradually corrode conductive layers, especially, aluminum lines, whereby the reliability and expected life of the semiconductor device are reduced.
Generally, a semiconductor device is provided with a covering layer 6 (FIG. 1) made of PSG containing a low concentration of phosphorus and having little moisture absorption. Furthermore, the semiconductor device is generally sealed in a package. Even so, there are still problems in reliability and expected life caused by the moisture absorption of the underlying layer 4 of PSG containing a high concentration of phosphorus. When a resin seal package is used, moisture inevitably penetrates from the air along the lead wires. The penetrated moisture reaches the underlying PSG layer 4 from around the exposed bonding pad portion of the conductive layer 5 after a long period, thereby causing corrosion of a portion of the conductive layer 5.